State diesel rules aimed at improving public health have also reduced levels of black carbon, a potent contributor to climate change, according to a new report commissioned by the California Air Resources Board.

The study was led by Veerabhadran Ramanathan, a professor of climate science at the Scripps Institution of Oceanography in La Jolla who collaborated with the Department of Energy’s Lawrence Berkeley National Laboratory and the Pacific Northwest National Laboratory in Richland, Wash.

California has targeted diesel pollution for decades, including a series of measures starting in 2000. One of the most controversial diesel mandates was approved in 2008, focusing on diesel-powered trucks and buses, as part of a sweeping set of regulations designed to meet federal air-quality standards and cut down on cases of respiratory illness and premature death. Supporters have hailed it as a major tool for battling asthma and cancer, while opponents have said it’s an overly aggressive measure that has cost many jobs.

Ramanathan and his colleagues said their conservative estimate shows the diesel requirements may slow climate change by curbing the equivalent of 21 million metric tons of carbon dioxide annually. That’s about the same as taking more than 4 million vehicles off the road every year, the report concluded.

“Diesel is a toxin and is high on our list of dangerous emissions, so we’re really getting a global warming benefit as well as a public health benefit as a result of reducing diesel exhaust,” said San Diego County Supervisor Ron Roberts, who has also served on the Air Resources Board during its years of discussion about diesel pollution.

There’s general consensus among scientists that the world’s climate is warming and that fossil-fuel combustion is likely the leading cause. While carbon dioxide is the main contributor, black carbon produced by diesel exhaust and other emissions is No. 2, Ramanathan said.

His globe-trotting work, from China to California, has examined short-lived greenhouse compounds including black carbon, which lasts two weeks in the atmosphere, compared to carbon dioxide, which can persist for centuries. So cutting diesel emissions can swiftly reduce the rate of climate change, he said. “If this can be replicated worldwide, we can reduce projected global warming by as much as 15 percent over the next 30 years,” Ramanathan said.

The Air Resources Board commissioned the report through a competitive-bid process, ultimately selecting Ramanathan and his collaborators.

To conduct the study, Ramanathan said, the scientists examined decades of black-carbon measurements. “From the 1980s, black carbon had come down 50 percent,” he said. “Since the 1960s, the amount of soot we put out came down by 90 percent.”

Diesel use rose three- to four-fold during the same period, he said, but improved fuel formulations and engine filters slashed the amount of black carbon emitted.

Early on, concentrations of black carbon dropped in California because of state rules controlling tailpipe emissions and the burning of trash and coal, said Melanie Turner, a spokeswoman for the board. Upcoming phases will address diesel exhaust from agricultural equipment, she said.

When the Air Resources Board voted unanimously in 2008 to adopt the diesel rules pertaining to diesel-powered vehicles, it sided with the agency’s research and a history of past studies by others linking diesel pollution to various health problems.

At the time, advocates of the regulations said the mandate to replace or retrofit up to 1 million trucks and buses would prevent tens of thousands of asthma cases and save up to $68 billion in lost work days during the next 15 years.

Industry officials didn’t dispute the health connections, but said the state’s actions were extreme because they didn’t adequately account for the economic impact on a wide range of businesses that use diesel-powered vehicles, from beekeepers in North County to long-haul truckers based in the San Joaquin Valley. They pointed to a widely agreed-upon projection that the rules would cost industry an estimated $5.5 billion by 2023.

The divisiveness grew about a year later, when the agency confirmed that the lead author of internal research about diesel pollution’s effects on public health had exaggerated his academic credentials. That individual was demoted but the Air Resources Board stayed its course, saying the fundamental health analysis remained sound.

Diesel technology pioneered in California could be readily adopted by industrialized nations, pollution experts said, but may be harder to introduce in developing countries, which rely on less efficient diesel-fueled cars and motorcycles.

“In developing countries, it’s always more of a challenge,” said Michel Boudrias, a professor of marine science and environmental studies and director of sustainability for the University of San Diego.

The equipment comes at a price, adding about $10,000 to the $100,000 price tag of a big-rig truck, said Allen Schaeffer, executive director of the Diesel Technology Forum, a trade organization based in Washington, D.C.

Such an expense is still less of a barrier than the availability of the right fuel, he said. New filters require more refined diesel formulations that aren’t found in developing countries, he explained.

“The technology is widely available in the U.S. and available for export, but it’s very difficult to achieve the gains we’ve had in the U.S. without cleaner fuel,” he said.

Despite disagreements about global feasibility and economic hardships, Dave De Haan, a University of San Diego chemistry professor who has researched particulate pollution, said the new report leaves little doubt about the need to rein in diesel emissions.

“This study was kind of a third strike against burning diesel in engines without emission controls,” he said. “We’ve known they generate C02, which warms the climate, and we know it generate particulates, which damage human health. What this study makes clear is that now we know those particulates are also bad for climate.”